The invention lies in the printing technology field. More specifically, the invention relates to an offset printing unit and a corresponding offset printing method.
In rotary offset printing, the image to be printed is offset from a form cylinder onto a rubber blanket, and it is subsequently offset from the rubber blanket to the surface to be printed. Typically, the form cylinder is a plate cylinder, i.e., a roller which carries a printing plate onto which ink and water is applied in accordance with the image pattern on the plate. The rubber blanket typically forms the peripheral surface of a rubber blanket cylinder with the same diameter as the plate cylinder.
Japanese patent application JP 7-266533 discloses a printing press in which the rubber blanket is formed by a blanket belt which is guided about two blanket rollers. The two blanket rollers have the same diameter as the plate cylinder. One of the guide rollers defines the offset nip between the plate cylinder and the blanket, and the other guide roller defines the printing nip (the line along which the ink is transferred onto the paper) between the blanket and the paper web. The object pursued by the Japanese disclosure, is to move the respective printing nips of the various printing units closer towards one another so as to reduce the disadvantageous effect of fan-out registration errors. Since, in the Japanese disclosure, the diameters of the blanket cylinders are equal to the diameters of the plate cylinder, and each of the printing units (each unit prints one color) thus require six such full-diameter cylinders for double-sided web printing, the construction costs for that printing press and the attendant space requirements are enormous.
German published patent application DE 44 42 983 discloses a sheetfed printing machine with two plate cylinders which transfer a first and a second color to a common blanket belt. The blanket belt is guided about two blanket cylinders with the same diameter as the plate cylinders. The system is used to reduce the number of transfer cylinders for the paper sheets, but it does not have a bearing on the fan-out registration problem. The number of necessary cylinders (i.e., plate cylinders and blanket cylinders) is not reduced relative to the conventional prior art machines and the space needed for the printing units is the same as in conventional sheetfed printing units with conventional rubber blanket rollers.
U.S. Pat. No. 5,907,997 discloses a satellite printing unit with four print and blanket cylinder couples that are arranged around a central impression cylinder. The system requires a substantial amount of space and, importantly, it can only be implemented with short ink trains and dampener trains. Moreover, the number of equal-diameter plate and blanket cylinders is not reduced.
The term fan-out registration is a term used in the art to describe the effect of paper web expansion during the printing operation. As the paper web travels through the individual printing units, a considerable amount of ink and water are offset onto the web. The water thereby causes the web to expand. The primary expansion, due to the prevalent longitudinal alignment of the paper fibers in the web, is in the lateral direction, i.e., transverse to the web travel direction. This can lead to considerable misregistration among the various printing units, with the misregistration between the color of the first unit and the color of the last unit naturally being the most noticeable. The fan-out problem becomes especially pronounced in very wide printing machines, such as the four page wide machines with rollers having an axial length of more that 60 inches.
Fan-out misregistration has conventionally been compensated for by a (predicted) shifting of the downline printing plates into better registration. That is, plate register pins are moved so that the position of the plate relative to the centerline of the printing machine will coincide with the predicted position of the preceding color after the expected amount of growth. Additionally, it has been known to subject the web to so-called bustle wheels just upline of each of the following printing units. The bustle wheels cause a slight crumpling and shrinking of the web just prior to its entry into the following printing nip (see, for example, U.S. Pat. Nos. 5,794,829 and 6,105,498).
These counter-measures are only static solutions which compensate for predicted errors. They do not account for dynamic changes in the press such as varying web properties or varying print coverages.
It is accordingly an object of the invention to provide a rotary offset printing machine with a rubber blanket belt, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which reduces or eliminates fan-out registration errors and is enabled to reduce or eliminate color registration errors during make-ready without causing paper waste.
With the foregoing and other objects in view there is provided, in accordance with the invention, an offset printing unit for printing a material web, comprising:
a first form cylinder for transferring a first color, the first form cylinder having a form cylinder diameter;
a second form cylinder for transferring a second color, the second form cylinder having the form cylinder diameter;
a plurality of guide rollers respectively disposed in vicinity of the first and second form cylinders and of a material web to be imprinted, the guide rollers having a diameter smaller that the form cylinder diameter and/or a mass that is less than that of the form cylinders; and
an endless blanket belt disposed to revolve around the guide rollers and to form with the first and second form cylinders respective offset nips at which the blanket belt receives the first color from the first form cylinder and the second color from the second form cylinder, and to form an impression nip with the material web at which the material web receives the first and second colors from the blanket belt.
In accordance with an added feature of the invention, the first and second form cylinders are plate cylinders.
In accordance with an additional feature of the invention, a third and a fourth form cylinder are disposed mirror-symmetrically to the first and second form cylinders across the material web, a further plurality of guide rollers disposed opposite the guide rollers across the material web, and a further endless blanket belt disposed to revolve about the further guide rollers, to form respective offset nips with the third and fourth form cylinders, and to offset colors received from the third and fourth form cylinders onto the material web across from the first above-mentioned impression nip.
In accordance with an alternative embodiment of the invention, a third and a fourth form cylinder disposed to form respective offset nips with the blanket belt for offsetting onto the blanket belt a third and a fourth color respectively, and whereby the blanket belt simultaneously imprints onto the material web up to four colors at the impression nip.
In accordance with another feature of the invention, the form cylinders have a diameter substantially twice the diameter of the guide rollers.
In accordance with a further feature of the invention, two H-type units are stacked one above the other, each comprising at least two form cylinders, and wherein the guide rollers are disposed to form respective impression nips of the two H-type units at a spacing distance less than twice the form cylinder diameter.
In accordance with a preferred embodiment of the invention, a camera is disposed at a location downline from the second form cylinder in a blanket belt travel direction and configured to record a coverage of the first and second color on the blanket belt, and a computer connected to the camera and to the form cylinders for setting a register of the first and second colors on the blanket belt in response to a signal received from the camera.
With the above and other objects in view there is also provided, in accordance with the invention, an offset printing method, which comprises:
providing an endless rubber blanket belt disposed to revolve about a plurality of guide rollers;
offsetting a plurality of colors onto the blanket belt at respective offset nips formed between the blanket belt and a plurality of form cylinders disposed along a travel path of the blanket belt;
ascertaining a register between the plurality of colors and continuing the offsetting step until register has been attained; and
after register between the plurality of colors has been attained, throwing on the blanket belt onto a material web and simultaneously printing the plurality of colors at a single impression nip formed between the blanket belt and the material web.
In accordance with again an added feature of the invention, four colors are offset onto each of two blanket belts disposed on opposite sides of the material web, and both sides of the web are simultaneously printed each with up to four colors.
In accordance with a concomitant feature of the invention, four blanket belts are provided and each is configured to have offset thereon at least two colors. In this case, the material web is imprinted with at least two of the four blanket belts at respective impression nips disposed in close vicinity along the material web.
The primary advantageous feature of the invention is thus the use of a single blanket from which at least two colors can be transferred from two form cylinders onto the print material. The problem of fan-out registration is therefore eliminated between the two colors. The blanket belt, furthermore, is guided about small-diameter guide rollers, which leads to a substantial savings in material cost and space requirement. The novel system no longer requires equal diameter plate and blanket cylinders and the system is not limited to short inkers.
It has been found to be a further, yet substantial, advantage of the invention, that it is possible with the new system to completely adjust the register of different colors on the blanket belt before the belt is brought into contact with the paper web. Accordingly, the color register can be completely during make-ready, without wasting any paper.
Due to the light-weight and smaller diameter construction of the guide rollers (two of which also form the impression rollers), it is possible to very quickly react to a web break. In that case, the low-mass rollers can be quickly retracted out of contact with the paper web.
It is yet a further advantage of the invention that the number of motors can be reduced. For example, in a five high printing tower (CMYB, plus an extra color) with three motor drives, it is possible to save four motors and the plate cylinders can be evenly spaced along the printing units.
Several auxiliary units can be dispensed with in accordance with the invention. For example, only a single blanket washer is required for each belt. If the belt is used as a four-color offset blanket, for example, three blanket washers can be eliminated as compared with the prior art system.
Finally, by utilizing segmented blanket belts which can be selectively lengthened and shortened, it is possible to change over among a variety of systems, as will become clear from the following description of the preferred embodiments.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a printing machine with a rubber blanket belt, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.